send_file has :x_sendfile param which defaults to true in Rails 3.
This feature offloads streaming download to front server - Apache (with mod_xsendfile) or lighttpd, by returning empty response with X-Sendfile header with path.
Nginx uses X-Accel-Redirect header for same functionality but you have to configure Rails properly in proper environment file:
config.action_dispatch.x_sendfile_header = 'X-Accel-Redirect'
Rails 3 update: this line already exists in production.rb, just uncomment it.
Add sendfile on; to your nginx config to utilize header sent by Rails. Remember the absolute path must be used and nginx must have read access to file.